The present invention relates to an anode including an anode current collector and an anode active material layer, and a battery using the anode.
In recent years, as mobile devices have higher performance and more functions, higher capacities of secondary batteries as power sources of the mobile devices have been desired. As a secondary battery which meets the requirement, a lithium secondary battery is utilized. However, the battery capacity of a currently typical lithium secondary battery which uses lithium cobalt oxide as a cathode and graphite as an anode has reached a point of saturation, so it is extremely difficult to substantially increase the capacity of the lithium secondary battery. Although an anode using metal lithium (Li) has been studied since a long time ago, in order to put the anode to practical use, it is required to improve precipitation/dissolution efficiency and control dendritic precipitation.
On the other hand, an anode with a high capacity which uses silicon (Si), tin (Sn) or the like has been actively studied recently. However, when charge and discharge are repeated, the anode is broken into small pieces due to severe expansion and shrinkage of an anode active material of the anode, thereby a current collecting property declines, or the decomposition of an electrolyte solution is accelerated due to an increase in a surface area, thereby cycle characteristics are extremely poor. Therefore, an anode in which an anode active material layer is formed on an anode current collector through a vapor-phase deposition method, a liquid-phase deposition method or a sintering method (for example, refer to Japanese Unexamined Patent Application Publication No. Hei 8-50922, Japanese Patent No. 2948205, and Japanese Unexamined Patent Application Publication No. Hei 11-135115) has been studied. Compared to a conventional coating type anode to which slurry including a particulate anode active material, a binder and the like is applied, the anode can be prevented from being broken into small pieces, and the anode current collector and the anode active material layer can be formed as one unit, so the electronic conductivity in the anode is extremely superior. Therefore, higher performance in terms of capacity and cycle lifespan is expected. Moreover, an electronic conductor, a binder and gaps which are present in a conventional anode can be reduced or eliminated, so the anode can be essentially formed into a thin film.
In the integral type anode, in the case where the anode active material layer includes tin, for example, heat treatment is preferably performed to accelerate alloying between the anode current collector and the anode active material layer in at least a portion of an interface between the anode current collector and the anode active material layer. Generally, the heat treatment is performed after an anode in which an anode active material layer is formed on a strip-shaped anode current collector is wound into a roll.
However, in this case, the anode active material layers formed on both sides of the anode current collector face and come into contact with each other, so the anode active material layers may be fusion bonded by the heat treatment. Therefore, a fracture in the anode may occur in a process of unfolding the anode wound into a roll and laminating the anode on a cathode, or cycle characteristics may be degraded due to unevenness in the thickness of the anode active material layer.